After nearly four years away, the 44-year-old American great partnered Canadian teenager Mboko to victory against Nicole Melichar-Martinez and Erin Routliffe in front of a sell-out crowd on Tuesday.
They were scheduled to play Canada's Leylah Fernandez and Germany's Laura Siegemund in the quarter-final, but 19-year-old Mboko has been forced to withdraw after slipping painfully during her singles match against Czech Republic's Karolina Pliskova on Wednesday.
Mboko had to retire from that match and left the court hobbling and close to tears.
Williams, who has won 23 Grand Slam singles titles, will continue her comeback in Berlin next week and could seek a wildcard for the Wimbledon championships.
